In the past, there has been extensive description in the patent and other technical literature of electrophoretic migration imaging processes. For example, a description of such processes may be found in U.S. Pat. Nos. 2,758,939 by Sugarman issued Aug. 14, 1956; 2,940,847, 3,100,426, 3,140,175 and 3,143,508, all by Kaprelian; 3,384,565, 3,384,488 and 3,615,558, all by Tulagin et al; 3,384,566 by Clark; and 3,383,993 by Yeh. In addition to the foregoing patent literature directed to conventional photoelectrophoretic migration imaging processes, another type of electrophoretic migration imaging process which advantageously provides for image reversal is described in Groner, U.S. Pat. No. 3,976,485 issued Aug. 24, 1976.
Certain important differences exist between the specific electrophoretic migration imaging processes described, for example, in the above-noted patents to Sugarman, Kaprelian, Tulagin et al, Clark and Yeh, all of which deal with conventional electrophoretic migration imaging processes, and the above-noted Groner U.S. patent. The Groner application describes a novel method and apparatus for obtaining image reversal as a consequence of the electrical interaction between unexposed photosensitive particles and a "dark charge exchange" electrode quite different from that which occurs in conventional electrophoretic migration imaging processes. However, there are certain general points of similarity existing in each of the electrophoretic migration imaging processes described in the foregoing patents.
In general, each of the foregoing electrophoretic migration imaging processes typically employs a layer of electrostatic charge-bearing photoconductive particles, i.e., electrically photosensitive particles, positioned between two spaced electrodes, one of which may be transparent. To achieve image formation in these processes, the charge-bearing photosensitive particles positioned between the two spaced electrodes, as described above, are subjected to the influence of an electric field and exposed to activating radiation. As a result, the charge-bearing electrically photosensitive particles are caused to migrate electrophoretically to the surface of one or the other of the spaced electrodes, and one obtains an image pattern on the surface of these electrodes. Typically, a negative image is formed on one electrode, and a positive image is formed on the opposite electrode. Image discrimination occurs in the various electrophoretic migration imaging processes as a result of a net change in charge polarity of either the exposed electrically photosensitive particles (in the case of conventional electrophoretic migration imaging) or the unexposed electrically photosensitive particles (in the case of the electrophoretic migration imaging process described in the above-noted Groner patent application) so that the image formed on one electrode surface is composed ideally of electrically photosensitive particles of one charge polarity, either negative or positive polarity, and the image formed on the opposite polarity electrode surface is composed ideally of electrically photosensitive particles having the opposite charge polarity, either positive or negative.
In any case, regardless of the particular electrophoretic migration imaging process employed, it is apparent that an essential component of any such process is the electrically photosensitive particles. And, of course, to obtain an easy-to-read, visible image it is important that these electrically photosensitive particles be colored, as well as electrically photosensitive. Accordingly, as is apparent from the technical literature regarding electrophoretic migration imaging processes, work has been carried on in the past and is continuing to find particles which possess both useful levels of electrical photosensitivity and which exhibit good colorant properties. Thus, for example, various types of electrically photosensitive materials are disclosed for use in electrophoretic migration imaging processes, for example, in U.S. Pat. Nos. 2,758,939 by Sugarman, 2,940,847 by Kaprelian, and 3,384,488 and 3,615,558 by Tulagin et al, noted hereinabove.
In large part, the art, to date, has generally selected useful electrically photosensitive or photoconductive pigment materials for electrophoretic migration imaging from known classes of photoconductive materials which may be employed in conventional photoconductive element, e.g., photoconductive plates, drums, or webs used in electrophotographic office-copier devices. For example, both Sugarman and Kaprelian in the above-referenced patents state that electrically photosensitive materials useful in electrophoretic migration imaging processes may be selected from known classes of photoconductive materials. And, the phthalocyanine pigments described as a useful electrically photosensitive material for electrophoretic imaging processes in U.S. Pat. No. 3,615,558 by Tulagin et al have long been known to exhibit useful photoconductive properties.
It is recognized, as set forth above, that many useful electrically photosensitive materials which are employed in electrophoretic migration imaging processes can be and have been selected from known photoconductive materials. However, in accord with the present invention it has unexpectedly been found after extensive investigation of one particular class of known photoconductive materials including, but not limited to, those materials described in U.S. Pat. No. 3,246,983 issued Apr. 19, 1966, 3,567,450 issued Mar. 2, 1971, 3,653,887 issued Apr. 4, 1972, and 3,873,312 issued Mar. 25, 1975, that a particular subclass of these materials within the larger class of organic photoconductive materials exemplified by the above-noted patents are highly useful in electrophoretic migration imaging processes as electrophotosensitive materials and/or as chemical sensitizers for other electrophotosensitive materials whereas many closely related materials within this same class of known organic photoconductive materials show little or no utility in electrophoretic migration imaging processes.